1. Field of the Invention
This invention is concerned with a jet propelled boat having an improved propulsion unit.
2. Description of the Prior Art
Jet propulsion units for boats have been used for many years and such units usually consist of a pump driven by a conventional gas engine. Water is drawn through an inlet into an impeller housing and is discharged by the impeller at the stern of the boat. The advantage of these jet propelled boats is that it is possible to have practically no moving parts outside of the boat, particularly extending beneath the bottom of the boat. Thus, such boats can be used in very weedy areas and in other areas where the boat might encounter foreign objects that might damage a propeller, for example. For one to realize the full advantage of a jet propulsion unit, it is desirable that the unit be very compact vertically. The unit should not project much if any below the bottom of the boat and preferably should not extend much above the top of the transom. This is particularly important in fishing where it is desirable to have the area above the hull as free from obstructions as possible. A typical type of boat in which a very compact jet propulsion unit is used is shown in the Arnold U.S. Pat. No. 3,889,623. Another patent of interest in connection with this is the Smith U.S. Pat. No. 3,785,327.
The problem arises, whenever use is made of a jet propulsion unit that is very compact vertically, that the engine tends to be either below or close to the surface of the normal water line. It is customary in such boats to discharge the exhaust gas below the surface of the water. Under these conditions, it is very easy, when the boat is being loaded prior to starting the engine, for water to enter the exhaust system and even into the exhaust manifold. This is due to the fact that there is very little vertical distance between the level at which the exhaust gas is discharged and the level of the exhaust manifold. Obviously, if water enters into the exhaust manifold, it can seriously interfere with the operation of the engine and certainly with the initial starting of the engine.
One attempt to avoid the problem in connection with jet engines for boats is shown in the Mattson et al U.S. Pat. No. 3,194,205. In this patent, a pipe leading from the exhaust passage is brought up over the top of the propulsion unit before entering the exhaust manifold. This presents several problems, in the first place, it places a very hot pipe up near the top of the engine where, if the engine cover is removed, it is apt to be engaged by an occupant of the boat. Furthermore, the provision of such a long pipe between the exhaust gas outlet and the exhaust manifold greatly increases the exhaust gas back pressure and hence decreases the efficiency of the engine.
In the Dawson U.S. Pat. No. 4,274,357, assigned to the same assignee as the present invention, there is shown a jet propulsion unit for operation of a surfboard. Because of the necessity of having a relatively compact unit, the distance between the engine and the impeller is relatively small. Dawson overcomes the problem of water entering through the exhaust gas outlet into the exhaust manifold by providing a cushion chamber around the impeller housing and through which the exhaust gas flows. Because of the large diameter of the cushion chamber, there is relatively little pressure drop. Furthermore, the chamber, being sealed tight, provides a very effective gas cushion to guard against entrance of water. There is no recognition in the Dawson Patent that the jet engine of that patent might solve the problems in connection with a propulsion unit for a conventional boat. I have discovered that an engine of the type shown in the Dawson Patent with a vertical shaft and a gas chamber around the impeller housing if used as a propulsion unit for a boat solves many of the problems previously encountered in connection with trying to provide a jet propulsion unit for a boat which is compact vertically and which does not allow water to enter the exhaust manifold system when the boat is standing still and someone is standing in it.